So you'd like to use your computer as a LANDLINE replacement ?
That's not actually possible in the classic sense of the word.
You could use VoIP as a solution despite what you may think
I don't have a net connection therefore VoIP is out of question
You clearly have a net connection otherwise you would NOT of been able to post this question at all.
For that to be possible you machine would need to be always on to accept any incoming calls.
Although using a product like Skype you can setup voice mail service for the times you are not online.